24 Ranges library [ranges]

24.6 Range factories [range.factories]

24.6.1 General [range.factories.general]

Subclause [range.factories] defines range factories, which are utilities to create a view.
Range factories are declared in namespace std​::​ranges​::​views.

24.6.2 Empty view [range.empty]

24.6.2.1 Overview [range.empty.overview]

empty_­view produces a view of no elements of a particular type.
[Example 1: auto e = views::empty<int>; static_assert(ranges::empty(e)); static_assert(0 == e.size()); — end example]

24.6.2.2 Class template empty_­view [range.empty.view]

namespace std::ranges { template<class T> requires is_object_v<T> class empty_view : public view_interface<empty_view<T>> { public: static constexpr T* begin() noexcept { return nullptr; } static constexpr T* end() noexcept { return nullptr; } static constexpr T* data() noexcept { return nullptr; } static constexpr size_t size() noexcept { return 0; } static constexpr bool empty() noexcept { return true; } }; }

24.6.3 Single view [range.single]

24.6.3.1 Overview [range.single.overview]

single_­view produces a view that contains exactly one element of a specified value.
The name views​::​single denotes a customization point object ([customization.point.object]).
Given a subexpression E, the expression views​::​single(E) is expression-equivalent to single_­view<decay_­t<decltype((E))>>(E).
[Example 1: for (int i : views::single(4)) cout << i; // prints 4 — end example]

24.6.3.2 Class template single_­view [range.single.view]

namespace std::ranges { template<copy_­constructible T> requires is_object_v<T> class single_view : public view_interface<single_view<T>> { private: copyable-box<T> value_; // exposition only (see [range.copy.wrap]) public: single_view() requires default_­initializable<T> = default; constexpr explicit single_view(const T& t); constexpr explicit single_view(T&& t); template<class... Args> requires constructible_­from<T, Args...> constexpr explicit single_view(in_place_t, Args&&... args); constexpr T* begin() noexcept; constexpr const T* begin() const noexcept; constexpr T* end() noexcept; constexpr const T* end() const noexcept; static constexpr size_t size() noexcept; constexpr T* data() noexcept; constexpr const T* data() const noexcept; }; template<class T> single_view(T) -> single_view<T>; }
constexpr explicit single_view(const T& t);
Effects: Initializes value_­ with t.
constexpr explicit single_view(T&& t);
Effects: Initializes value_­ with std​::​move(t).
template<class... Args> requires constructible_­from<T, Args...> constexpr explicit single_view(in_place_t, Args&&... args);
Effects: Initializes value_­ as if by value_­{in_­place, std​::​forward<Args>(args)...}.
constexpr T* begin() noexcept; constexpr const T* begin() const noexcept;
Effects: Equivalent to: return data();
constexpr T* end() noexcept; constexpr const T* end() const noexcept;
Effects: Equivalent to: return data() + 1;
static constexpr size_t size() noexcept;
Effects: Equivalent to: return 1;
constexpr T* data() noexcept; constexpr const T* data() const noexcept;
Effects: Equivalent to: return value_­.operator->();

24.6.4 Iota view [range.iota]

24.6.4.1 Overview [range.iota.overview]

iota_­view generates a sequence of elements by repeatedly incrementing an initial value.
The name views​::​iota denotes a customization point object ([customization.point.object]).
Given subexpressions E and F, the expressions views​::​iota(E) and views​::​iota(E, F) are expression-equivalent to iota_­view(E) and iota_­view(E, F), respectively.
[Example 1: for (int i : views::iota(1, 10)) cout << i << ' '; // prints: 1 2 3 4 5 6 7 8 9 — end example]

24.6.4.2 Class template iota_­view [range.iota.view]

namespace std::ranges { template<class I> concept decrementable = see below; // exposition only template<class I> concept advanceable = see below; // exposition only template<weakly_­incrementable W, semiregular Bound = unreachable_sentinel_t> requires weakly-equality-comparable-with<W, Bound> && copyable<W> class iota_view : public view_interface<iota_view<W, Bound>> { private: // [range.iota.iterator], class iota_­view​::​iterator struct iterator; // exposition only // [range.iota.sentinel], class iota_­view​::​sentinel struct sentinel; // exposition only W value_ = W(); // exposition only Bound bound_ = Bound(); // exposition only public: iota_view() requires default_­initializable<W> = default; constexpr explicit iota_view(W value); constexpr iota_view(type_identity_t<W> value, type_identity_t<Bound> bound); constexpr iota_view(iterator first, see below last); constexpr iterator begin() const; constexpr auto end() const; constexpr iterator end() const requires same_­as<W, Bound>; constexpr auto size() const requires see below; }; template<class W, class Bound> requires (!is-integer-like<W> || !is-integer-like<Bound> || (is-signed-integer-like<W> == is-signed-integer-like<Bound>)) iota_view(W, Bound) -> iota_view<W, Bound>; }
Let IOTA-DIFF-T(W) be defined as follows:
  • If W is not an integral type, or if it is an integral type and sizeof(iter_­difference_­t<W>) is greater than sizeof(W), then IOTA-DIFF-T(W) denotes iter_­difference_­t<W>.
  • Otherwise, IOTA-DIFF-T(W) is a signed integer type of width greater than the width of W if such a type exists.
  • Otherwise, IOTA-DIFF-T(W) is an unspecified signed-integer-like type ([iterator.concept.winc]) of width not less than the width of W.
    [Note 1:
    It is unspecified whether this type satisfies weakly_­incrementable.
    — end note]
The exposition-only decrementable concept is equivalent to:
template<class I> concept decrementable = incrementable<I> && requires(I i) { { --i } -> same_­as<I&>; { i-- } -> same_­as<I>; };
When an object is in the domain of both pre- and post-decrement, the object is said to be decrementable.
Let a and b be equal objects of type I.
I models decrementable only if
  • If a and b are decrementable, then the following are all true:
  • If a and b are incrementable, then bool(--(++a) == b).
The exposition-only advanceable concept is equivalent to:
template<class I> concept advanceable = decrementable<I> && totally_­ordered<I> && requires(I i, const I j, const IOTA-DIFF-T(I) n) { { i += n } -> same_­as<I&>; { i -= n } -> same_­as<I&>; I(j + n); I(n + j); I(j - n); { j - j } -> convertible_­to<IOTA-DIFF-T(I)>; };
Let D be IOTA-DIFF-T(I).
Let a and b be objects of type I such that b is reachable from a after n applications of ++a, for some value n of type D.
I models advanceable only if
  • (a += n) is equal to b.
  • addressof(a += n) is equal to addressof(a).
  • I(a + n) is equal to (a += n).
  • For any two positive values x and y of type D, if I(a + D(x + y)) is well-defined, then I(a + D(x + y)) is equal to I(I(a + x) + y).
  • I(a + D(0)) is equal to a.
  • If I(a + D(n - 1)) is well-defined, then I(a + n) is equal to [](I c) { return ++c; }(I(a + D(n - 1))).
  • (b += -n) is equal to a.
  • (b -= n) is equal to a.
  • addressof(b -= n) is equal to addressof(b).
  • I(b - n) is equal to (b -= n).
  • D(b - a) is equal to n.
  • D(a - b) is equal to D(-n).
  • bool(a <= b) is true.
constexpr explicit iota_view(W value);
Preconditions: Bound denotes unreachable_­sentinel_­t or Bound() is reachable from value.
Effects: Initializes value_­ with value.
constexpr iota_view(type_identity_t<W> value, type_identity_t<Bound> bound);
Preconditions: Bound denotes unreachable_­sentinel_­t or bound is reachable from value.
When W and Bound model totally_­ordered_­with, then bool(value <= bound) is true.
Effects: Initializes value_­ with value and bound_­ with bound.
constexpr iota_view(iterator first, see below last);
Effects: Equivalent to:
  • If same_­as<W, Bound> is true, iota_­view(first.value_­, last.value_­).
  • Otherwise, if Bound denotes unreachable_­sentinel_­t, iota_­view(first.value_­, last).
  • Otherwise, iota_­view(first.value_­, last.bound_­).
Remarks: The type of last is:
constexpr iterator begin() const;
Effects: Equivalent to: return iterator{value_­};
constexpr auto end() const;
Effects: Equivalent to: if constexpr (same_­as<Bound, unreachable_sentinel_t>) return unreachable_sentinel; else return sentinel{bound_};
constexpr iterator end() const requires same_­as<W, Bound>;
Effects: Equivalent to: return iterator{bound_­};
constexpr auto size() const requires see below;
Effects: Equivalent to: if constexpr (is-integer-like<W> && is-integer-like<Bound>) return (value_ < 0) ? ((bound_ < 0) ? to-unsigned-like(-value_) - to-unsigned-like(-bound_) : to-unsigned-like(bound_) + to-unsigned-like(-value_)) : to-unsigned-like(bound_) - to-unsigned-like(value_); else return to-unsigned-like(bound_ - value_);
Remarks: The expression in the requires-clause is equivalent to: (same_­as<W, Bound> && advanceable<W>) || (integral<W> && integral<Bound>) || sized_­sentinel_­for<Bound, W>

24.6.4.3 Class iota_­view​::​iterator [range.iota.iterator]

namespace std::ranges { template<weakly_­incrementable W, semiregular Bound> requires weakly-equality-comparable-with<W, Bound> && copyable<W> struct iota_view<W, Bound>::iterator { private: W value_ = W(); // exposition only public: using iterator_concept = see below; using iterator_category = input_iterator_tag; // present only if W models incrementable using value_type = W; using difference_type = IOTA-DIFF-T(W); iterator() requires default_­initializable<W> = default; constexpr explicit iterator(W value); constexpr W operator*() const noexcept(is_nothrow_copy_constructible_v<W>); constexpr iterator& operator++(); constexpr void operator++(int); constexpr iterator operator++(int) requires incrementable<W>; constexpr iterator& operator--() requires decrementable<W>; constexpr iterator operator--(int) requires decrementable<W>; constexpr iterator& operator+=(difference_type n) requires advanceable<W>; constexpr iterator& operator-=(difference_type n) requires advanceable<W>; constexpr W operator[](difference_type n) const requires advanceable<W>; friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_­comparable<W>; friend constexpr bool operator<(const iterator& x, const iterator& y) requires totally_­ordered<W>; friend constexpr bool operator>(const iterator& x, const iterator& y) requires totally_­ordered<W>; friend constexpr bool operator<=(const iterator& x, const iterator& y) requires totally_­ordered<W>; friend constexpr bool operator>=(const iterator& x, const iterator& y) requires totally_­ordered<W>; friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires totally_­ordered<W> && three_­way_­comparable<W>; friend constexpr iterator operator+(iterator i, difference_type n) requires advanceable<W>; friend constexpr iterator operator+(difference_type n, iterator i) requires advanceable<W>; friend constexpr iterator operator-(iterator i, difference_type n) requires advanceable<W>; friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires advanceable<W>; }; }
iterator​::​iterator_­concept is defined as follows:
  • If W models advanceable, then iterator_­concept is random_­access_­iterator_­tag.
  • Otherwise, if W models decrementable, then iterator_­concept is bidirectional_­iterator_­tag.
  • Otherwise, if W models incrementable, then iterator_­concept is forward_­iterator_­tag.
  • Otherwise, iterator_­concept is input_­iterator_­tag.
[Note 1:
Overloads for iter_­move and iter_­swap are omitted intentionally.
— end note]
constexpr explicit iterator(W value);
Effects: Initializes value_­ with value.
constexpr W operator*() const noexcept(is_nothrow_copy_constructible_v<W>);
Effects: Equivalent to: return value_­;
[Note 2:
The noexcept clause is needed by the default iter_­move implementation.
— end note]
constexpr iterator& operator++();
Effects: Equivalent to: ++value_; return *this;
constexpr void operator++(int);
Effects: Equivalent to ++*this.
constexpr iterator operator++(int) requires incrementable<W>;
Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;
constexpr iterator& operator--() requires decrementable<W>;
Effects: Equivalent to: --value_; return *this;
constexpr iterator operator--(int) requires decrementable<W>;
Effects: Equivalent to: auto tmp = *this; --*this; return tmp;
constexpr iterator& operator+=(difference_type n) requires advanceable<W>;
Effects: Equivalent to: if constexpr (is-integer-like<W> && !is-signed-integer-like<W>) { if (n >= difference_type(0)) value_ += static_cast<W>(n); else value_ -= static_cast<W>(-n); } else { value_ += n; } return *this;
constexpr iterator& operator-=(difference_type n) requires advanceable<W>;
Effects: Equivalent to: if constexpr (is-integer-like<W> && !is-signed-integer-like<W>) { if (n >= difference_type(0)) value_ -= static_cast<W>(n); else value_ += static_cast<W>(-n); } else { value_ -= n; } return *this;
constexpr W operator[](difference_type n) const requires advanceable<W>;
Effects: Equivalent to: return W(value_­ + n);
friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_­comparable<W>;
Effects: Equivalent to: return x.value_­ == y.value_­;
friend constexpr bool operator<(const iterator& x, const iterator& y) requires totally_­ordered<W>;
Effects: Equivalent to: return x.value_­ < y.value_­;
friend constexpr bool operator>(const iterator& x, const iterator& y) requires totally_­ordered<W>;
Effects: Equivalent to: return y < x;
friend constexpr bool operator<=(const iterator& x, const iterator& y) requires totally_­ordered<W>;
Effects: Equivalent to: return !(y < x);
friend constexpr bool operator>=(const iterator& x, const iterator& y) requires totally_­ordered<W>;
Effects: Equivalent to: return !(x < y);
friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires totally_­ordered<W> && three_­way_­comparable<W>;
Effects: Equivalent to: return x.value_­ <=> y.value_­;
friend constexpr iterator operator+(iterator i, difference_type n) requires advanceable<W>;
Effects: Equivalent to: return i += n;
friend constexpr iterator operator+(difference_type n, iterator i) requires advanceable<W>;
Effects: Equivalent to: return i + n;
friend constexpr iterator operator-(iterator i, difference_type n) requires advanceable<W>;
Effects: Equivalent to: return i -= n;
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires advanceable<W>;
Effects: Equivalent to: using D = difference_type; if constexpr (is-integer-like<W>) { if constexpr (is-signed-integer-like<W>) return D(D(x.value_) - D(y.value_)); else return (y.value_ > x.value_) ? D(-D(y.value_ - x.value_)) : D(x.value_ - y.value_); } else { return x.value_ - y.value_; }

24.6.4.4 Class iota_­view​::​sentinel [range.iota.sentinel]

namespace std::ranges { template<weakly_­incrementable W, semiregular Bound> requires weakly-equality-comparable-with<W, Bound> && copyable<W> struct iota_view<W, Bound>::sentinel { private: Bound bound_ = Bound(); // exposition only public: sentinel() = default; constexpr explicit sentinel(Bound bound); friend constexpr bool operator==(const iterator& x, const sentinel& y); friend constexpr iter_difference_t<W> operator-(const iterator& x, const sentinel& y) requires sized_­sentinel_­for<Bound, W>; friend constexpr iter_difference_t<W> operator-(const sentinel& x, const iterator& y) requires sized_­sentinel_­for<Bound, W>; }; }
constexpr explicit sentinel(Bound bound);
Effects: Initializes bound_­ with bound.
friend constexpr bool operator==(const iterator& x, const sentinel& y);
Effects: Equivalent to: return x.value_­ == y.bound_­;
friend constexpr iter_difference_t<W> operator-(const iterator& x, const sentinel& y) requires sized_­sentinel_­for<Bound, W>;
Effects: Equivalent to: return x.value_­ - y.bound_­;
friend constexpr iter_difference_t<W> operator-(const sentinel& x, const iterator& y) requires sized_­sentinel_­for<Bound, W>;
Effects: Equivalent to: return -(y - x);

24.6.5 Istream view [range.istream]

24.6.5.1 Overview [range.istream.overview]

basic_­istream_­view models input_­range and reads (using operator>>) successive elements from its corresponding input stream.
[Example 1: auto ints = istringstream{"0 1 2 3 4"}; ranges::copy(ranges::istream_view<int>(ints), ostream_iterator<int>{cout, "-"}); // prints 0-1-2-3-4- — end example]

24.6.5.2 Class template basic_­istream_­view [range.istream.view]

namespace std::ranges { template<class Val, class CharT, class Traits> concept stream-extractable = // exposition only requires(basic_istream<CharT, Traits>& is, Val& t) { is >> t; }; template<movable Val, class CharT, class Traits> requires default_­initializable<Val> && stream-extractable<Val, CharT, Traits> class basic_istream_view : public view_interface<basic_istream_view<Val, CharT, Traits>> { public: constexpr explicit basic_istream_view(basic_istream<CharT, Traits>& stream); constexpr auto begin() { *stream_ >> value_; return iterator{*this}; } constexpr default_sentinel_t end() const noexcept; private: struct iterator; // exposition only basic_istream<CharT, Traits>* stream_; // exposition only Val value_; // exposition only }; }
constexpr explicit basic_istream_view(basic_istream<CharT, Traits>& stream);
Effects: Initializes stream_­ with addressof(stream).
constexpr default_sentinel_t end() const noexcept;
Effects: Equivalent to: return default_­sentinel;
template<class Val, class CharT, class Traits> basic_istream_view<Val, CharT, Traits> istream_view(basic_istream<CharT, Traits>& s);
Effects: Equivalent to: return basic_­istream_­view<Val, CharT, Traits>{s};

24.6.5.3 Class template basic_­istream_­view​::​iterator [range.istream.iterator]

namespace std::ranges { template<movable Val, class CharT, class Traits> requires default_­initializable<Val> && stream-extractable<Val, CharT, Traits> class basic_istream_view<Val, CharT, Traits>::iterator { // exposition only public: using iterator_concept = input_iterator_tag; using difference_type = ptrdiff_t; using value_type = Val; constexpr explicit iterator(basic_istream_view& parent) noexcept; iterator(const iterator&) = delete; iterator(iterator&&) = default; iterator& operator=(const iterator&) = delete; iterator& operator=(iterator&&) = default; iterator& operator++(); void operator++(int); Val& operator*() const; friend bool operator==(const iterator& x, default_sentinel_t); private: basic_istream_view* parent_; // exposition only }; }
constexpr explicit iterator(basic_istream_view& parent) noexcept;
Effects: Initializes parent_­ with addressof(parent).
iterator& operator++();
Effects: Equivalent to: *parent_->stream_ >> parent_->value_; return *this;
void operator++(int);
Effects: Equivalent to ++*this.
Val& operator*() const;
Effects: Equivalent to: return parent_­->value_­;
friend bool operator==(const iterator& x, default_sentinel_t);
Effects: Equivalent to: return !*x.parent_­->stream_­;